I have been looking through the API documentation and cant seem to find an API for the Company List feature that parallels the Contact List API. At first I assumed that I could use the Contact List API to get data out of my Company Lists but when I hit the contats/v1/lists endpoint I only got information about the Contact List.
Currently, get company list via the HubSpot API is not supported.
Moving forward, if you haven't already, I'd recommend jumping over to the HubSpot Ideas Forum and posting this idea there so that it's visible to the HubSpot product team and other HubSpot customers and developers. Hopefully this will gather more interest.